I have been wondering this for a while, but I still am not sure yet about the answer. I have always read that the histogram extend from one side of the graph to the other in order to get the full amount of details and to capture the most light. But I have noticed that much of the time, my picture either looks washed out (overexposed) if the graph extendeds all the way to the right, or may look most realistic (look the same way as I see it in front of me) if it is only going from the left edge to the middle or just to the right of the middle. So my question is, is a histogram always supposed to extend to the edges, and if not is there some kind of rule that can be followed to know when to not take it to the edge? Maybe something like if it is a low light situation, the graph will be bunched from the middle to the left, in bright situations there may be no shadows, or just go with what looks best on the LCD?
#1. "RE: Histograms" In response to Reply # 0 Sun 03-Jul-11 05:27 AM by aolander
A histogram won't necessarily extend from one side to the other. If there is a wide range of brightness in the scene, e.g. dark shadows to bright light, it will, or it may even be clipped on one end or the other, or both. A scene with low contrast may only have a spike in the middle. Changing the exposure for the scene will push the graph to the right (with more exposure) or left (with less exposure).
The practice of "exposing to the right" is often used when shooting RAW as this gives the file the most data. The image may look too bright, but the exposure can be corrected in post processing to the level you want. This technique applies mostly to RAW, not so much if you shoot JPEGs.
The histogram represents the range and relative intensity of the different tones present in the image. If you take a picture of a monochromatic scene with flat lighting you will get a histogram that is a single spike in the center (in the center because the camera's meter will try to make it average to neutral gray.) If there are no shadows there will be no information in the left side of the display. If there is nothing white present in the scene there will be no information on the right side.
It is usually desirable to have a full histogram because it typically means you have a well exposed image with a wide range of tonality present. There are tricky situations where the camera meter will not make the best choice. It's your job to add some exposure compensation. Looking at the histogram will guide your choices.
"It is usually desirable to have a full histogram because it typically means you have a well exposed image with a wide range of tonality present."
That is not entirely true. A low contrast scene will never have a full histogram no matter how you expose it. The spike can be at the left end, in the middle, or at the right end depending on how you expose it, but it will never extend from left to right, and it still can be correctly exposed. You perhaps can boost the contrast up in the camera or in post processing set the black and white points to extend the histogram, but it won't look like the scene you started with.
Another way to think about histograms is: Pixel Count.
The left side of the histo is 'dark', the right is 'light'. The camera goes through the image, and counts how many dark, medium and light pixels there are. It then creates a graph showing you how many dark, medium and light pixels are in the photo.
In a perfect world, there would be no pixels at or near either end of the graph. All the pixels would fall between the two ends. But that is something that is difficult to achieve in the natural world (studio-type setups are exempt from this).
For an 'average' scene, you want the bulk of your pixels to fall as close to the middle of the histo as possible. "High key" and "Low key" shots are an exception.